Electric power distribution is a necessary component of systems that operate with electronic power or in the distribution of electronic power. For example, most electronic equipment is a load that is connected to a utility grid, wherein power arrives in one form and is transferred and transformed into a form more suitable for the equipment.
The distribution of electric power from utility companies to households and businesses utilizes a network of utility lines connected to each residence and business. The network or grid is interconnected with various generating stations and substations that supply power to the various loads and that monitor the lines for problems. Distributed electric power generation, for example, converting power from photovoltaic devices, micro-turbines, or fuel cells at customer sites, can function in conjunction with the grid. Loads that are connected to the grid take the generated power and convert it to a usable form while excess power can supplement the grid.
An electric utility grid generally can also consist of many independent energy sources energizing the grid and providing power to the loads on the grid. This distributed power generation is becoming more common throughout the world as alternative energy sources are being used for the generation of electric power. In the United States, the deregulation of electric companies has spurred the development of independent energy sources co-existing with the electric utility. Rather than have completely independent energy sources for a particular load, these alternative energy sources can tie into the grid and are used to supplement the capacity of the electric utility.
The number and types of independent energy sources is growing rapidly, and can include photovoltaic devices, wind, hydro, fuel cells, storage systems such as battery, super-conducting, flywheel and capacitor types, and mechanical means including conventional and variable speed diesel or internal combustion (IC) engines, Stirling engines, gas turbines, and micro-turbines. In many cases these energy sources can sell the utility company excess power from their source that is utilized on their grid.
However, each of these independent energy sources needs some type of power converter that feeds energy to the grid or used to directly power the various loads. There must also be some means to provide protection when the grid becomes unstable. In most scenarios the utility company is still the main power source and in many cases controls the independent source to some extent.
Whether a power system is used to provide power to a load or a grid, the efficiency of the power system is typically important. The fuel consumption and the emissions should be minimized in all cases.
A problem with the state of the art systems is that typical internal combustion (IC) engine generator systems must rotate at a fixed speed to provide a fixed frequency output. This dramatically limits the engines maximum output or overload power, decreases part-load fuel efficiency, and consequently increases emissions/KWhr of power produced.
Another problem with the existing systems is that the distribution system is subject to non-linear, high harmonic content and unbalanced loading. This is especially true where the distributed generation system operates independent of the utility grid, and must therefore provide all of the load required harmonic currents. In distributed power applications, high harmonic content or unbalanced loads may lead to utility grid instability, resonances or other unanticipated distribution system behavior that may cause catastrophic failure of the distribution system components. Such a failure can result in damage to equipment and possibly personal injury.
Power converters, such as inverters, are necessary in modern power systems for the energy generating devices such as photovoltaic devices, micro-turbines, variable speed IC engines, fuel cells, and superconducting storage systems. In general, these systems generate AC or DC electricity that needs to be converted to a conditioned AC for feeding into the power grid or for direct connection to loads.
Grid independent DC-AC inverters generally behave as sinusoidal voltage sources that provide power directly to the loads. This type of power distribution architecture is generally required to provide power to both 3-phase and single-phase, or line to neutral connected loads. Typically, 3-phase power inverters meet the 3-phase plus neutral requirement by isolating the power inverter from the loads with a delta-wye power transformer.
Grid connected AC inverters generally behave as a current source that injects a controlled AC sine wave current into the utility line. The controlled AC current is generated in sync with the observed utility zero crossings, and may be exactly in phase, generating at unity power factor where upon real power only is exported. It is also possible to generate a variable amount out of phase—at other than unity power factor—where real and reactive power is exported to the grid. An effective change in reactive power output can be made by either phase shifting the output current waveform with respect to voltage or by creating an asymmetric distortion to the output current waveform.
Whether grid connected or grid independent, typical generators demonstrate poor output waveform total harmonic distortion (THD) when connected to any non-linear loads. This is particularly true in the case of even order harmonic currents (2nd, 4th, 6th, 8th etc.). More specifically, typical generators and power transformers common to most power distribution systems demonstrate a tendency to saturate especially when exposed to even order or DC content, load generated non-linear currents. This causes the generator output voltage waveform to rapidly degrade while simultaneously increasing generator losses and operating temperatures, and decreasing the power actually coupled from the engine to the electrical load. A variety of factors define how steep this saturation transition will occur, including magnetic core material and construction, magnitude and frequency of harmonics, and generator operating temperature. At the least, very poor output power quality, nuisance circuit breaker tripping, increased distribution system component loss and increased operating temperatures will be observed.
Although generator or transformer saturation is not as likely to occur in utility grid connected systems, primarily due to the utility grid's typically lower impedance than the grid connected inverter system, distortion and instability may still occur. This problem is greatly aggravated where generators, or transformer isolated power inverters act as “stand alone” voltage sources, where the generator or inverter comprises the only power source to the local distribution system.
These problems are currently solved in the distribution system by over sizing the generator or distribution transformers. For power inverters, expensive gapped core type isolation transformers are commonly employed to decrease the power conditioning system susceptibility to even order harmonic currents, as well as isolate inverter generated DC voltage offsets from the distribution system. The increased cost and space requirements for the isolation transformers are problems that are well known in the industry.
Inverters that perform an AC conversion function and are connected to the grid are known as “Utility-Interactive Inverters”. Such inverters are the subject of several U.S. and international codes and standards, such as, the National Electrical Code, Article 690—Photovoltaic Systems; UL 1741, Standard for Photovoltaic Inverters; and IEEE 929—Recommended Practice for Utility Interface of Photovoltaic (PV) Systems.
One class of inverters, Pulse width modulator (PWM) inverters, is used in three-phase bridges, H-bridges, and half-bridge configurations. The bus capacitors, typically electrolytic, consist of two or more capacitors connected in series that are fed from a passive rectifier or actively switched front-end section.
In order to reduce the aforementioned problems, attempts have been made to produce an improved generator speed control and electronic power dispensing system. The present systems have general shortcomings and do not adequately address the aforementioned problems.
What is needed is a means of efficiently operating a variable speed generator with a fixed frequency electronic power inverter, at the optimum engine speed for a given load. This speed versus load curve may be optimized to develop the lowest possible emissions, highest possible efficiency, or even to provide the fastest transient response, or highest overload capability. This design must also be cost effective to manufacture and implement, and allow for easy incorporation into current designs.